The present invention relates generally to apparatus for reproducing multichannel record discs, and more particularly to apparatus for demodulating and reproducing an angle-modulated wave signal picked up from a multichannel record disc. The higher harmonic components are removed from the carrier wave center frequency, of the angle-modulated wave signal, contained within the picked up signal.
In general, a recording apparatus multiplexes a direct wave signal and an angle-modulated wave signal. The resulting multichannel signal is cut on a record disc. The cutter head used for making this recording has a relatively good crosstalk characteristic between the left and right channels, up to about the upper frequency limit of the angle-modulated signal. For example, a carrier wave center frequency, of the angle-modulated wave signal, may be 30 KHz and a transmission frequency band width may be 20 KHz to 45 KHz. The cutter speed and other variable are so selected that the cross-talk characteristic will be good up to about 45 KHz.
However, this crosstalk characteristic of the cutter head itself is poor. In general, it does not respond well in a frequency band higher than the band upper limit frequency, for example, in a frequency band higher than 50 KHz. For this reason, there often is a crosstalk between the left and right channels. When the higher harmonic components have frequencies which are two, three or more times the center frequency of the carrier wave the angle-modulated signal of one channel may also become recorded in the other channel. That is, in the cited frequency range, components of frequencies such as 60 KHz and 90 KHz are also recorded.
On the other hand, there are pickup cartridges for use in the apparatus which have reproducing characteristics which can reproduce signals of frequency bands up to about 90 KHz. When reproduction is carried out with such a pickup cartridge, even the unwanted frequency components such as 60 KHz and 90 KHz, are picked up.
In the reproducing apparatus, the multiplexed signal of the direct wave signal and the angle-modulated wave signal is picked up and supplied, respectively, to a low-pass filter and a band-pass filter. There, it is separated into the direct wave signal and the angle-modulated wave signal. The angle-modulated wave signal, thus separated, is supplied to a demodulating circuit, where it is demodulated.
In this case, it is necessary for the band-pass filter to have a characteristic which passes angle-modulated wave signals in the 20 KHz to 45 KHz band and to separate them from direct wave signals having frequencies lower than 15 KHz. For this reason, the filtering frequency characteristic has a dip in the vicinity of 18 KHz. In order to obtain a good group delay characteristic, it has a dip in the vicinity of 50 KHz to 52 KHz (or 50 KHz), in a symmetrical relationship to the first dip, also in the high-frequency range. Accordingly, the filtering frequency characteristic of this band-pass filter has side lobes at frequency positions such as 15 KHz, 60 KHz, and 90 KHz.
The unwanted frequency components of 60 KHz, 90 KHz, etc., produced by the crosstalk were recorded and, therefore, are contained in the picked up signal. These signals are attenuated to a certain level by the side lobe characteristic of the band-pass filter. They are passed and supplied to the demodulating circuit. At present, however, a phase locked loop (PLL) is generally used for demodulating the signals picked up from a multichannel record disc reproducing apparatus. Since this PLL is capable of detecting these frequency components 60 KHz, 90 KHz, etc., they influence the other channels as crosstalk. Consequently, when such frequency components of 60 KHz, 90 KHz, etc., are supplied to the demodulating circuit, beats are generated between these frequency components and the carrier wave center frequency of 30 KHz of the angle-modulated signal. A distortion, due to interference, is produced in the demodulated output, whereby an abnormal sound is generated.